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The energy balance of a photoinduced electron transfer reaction is given by the Rehm-Weller equation which combines the oxidation potential Eox(D) of the electron donor, the reduction potential Ered(A) of the electron acceptor, an electrostatic correction term C and the excited state energy of the light-absorbing species: It is shown that if light carries a thermodynamic entropy the excitation energy term must be given by ηE*, η being the efficiency of the conversion of the energy of light into chemical free energy. Measurements of fluorescence quenching through electron transfer at very low light intensities show that the Rehm-Weller equation remains valid in spite of its implied assumption that η = 1; it is concluded that contrary to much current thinking light is a form of high grade energy which can be converted in principle entirely into chemical free energy and electrical energy.
  • Temperature Effect on Back Electron-Transfer Reactions within a Geminate Radical Pair: the Influence of the Solvent on the Adiabaticity of the Process
    E. Vauthey and P. Suppan
    Chemical Physics, 139 (2-3) , 1989, p381-390
    DOI:10.1016/0301-0104(89)80150-8 | unige:3064 | Abstract | Article PDF
A study of the temperature dependence (from 233 to 353 K) of the rate of back electron-transfer reactions within geminate radical pairs by measurement of the free radical yield is reported. The radical pair is generated by photoinduced electron transfer with rhodamine 6G and oxazine 118 cations as electron acceptors and aromatic amines and methoxy-benzene derivatives as electron donors in acetonitrile, methanol and ethanol. In acetonitrile, the back electron transfer is non-adiabatic and apparent negative activation energies are observed for barrierless reactions. In alcohol solvents, an anomalously large temperature dependence is observed, which is attributed to a solvent-controlled adiabatic behaviour.

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Eric Vauthey

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